One in eight women will develop breast cancer in their lifetime and it is the most common cancer in women. It is recommended that radiotherapy treatment is delivered after initial surgery for breast cancer to substantially reduce the risk of site specific relapse. However, during breast cancer treatment using radiotherapy, the other breast (the contralateral breast) receives radiation dose as an unwanted side effect of the treatment. The association between low dose from peripheral ionizing radiation and the risk for secondary cancer has attracted interest specifically for the long-term surviving patients. Specifically, concerns regarding oncogenesis and second cancer induction are realized and invoke the need for ALARA (As Low As Reasonably Achievable) principles to be followed.
During radiation therapy, regardless of the treatment technique, the surrounding normal tissue outside the treated area inevitably receives some amount of radiation dose. Such dose outside the geometric boundaries of the treatment fields is known as peripheral dose. There are three main sources of peripheral dose: (a) leakage through the treatment collimation (x-rays); (b) scattered radiation from the secondary collimators and beam modifiers such as the MultiLeaf Collimator (MLC), physical wedges (x-rays and electrons); and (c) internal scatter originating in the patient (x-rays). It has been shown that peripheral doses can be as large as 20% of maximum dose for normally incident beams and that these values can increase with oblique angle of incidence.
To minimize radiation doses delivered to the contralateral breast, lung, and heart, some patients can be treated with a prone technique. If a supine treatment is used, to reduce contralateral breast dose, different types of shielding devices, and delivery techniques have been used. These include mobile high-density lead shields placed between the treatment machine and the patient. Other devices used were tissue-density superflab material laid over the patient's contralateral breast. Although these methods did reduce contralateral breast dose, they presented technical difficulties in their usage. Mobile lead shields need to be placed appropriately between the patient and the treatment head of the linear accelerator (linac). Such techniques are not very efficient since they demand precise positioning alignments. They also suffer from not being able to be shaped around the treatment field edges. Superflab bolus can also reduce skin and subcutaneous dose but it requires at least 10 mm thickness of bolus material to provide sufficient attenuation. This process may also introduce misalignment errors near the edge of the treatment fields.
What is required is an improved shielding method and device.